a. Field of the Invention
The present invention relates to methods and apparatus for making tanks from plates.
B. Description of the Prior Art
Various methods of construction and devices are already known to form storage tanks or silos from individual sheet-metal plates or panels or from unwound wide strip according to the so-called spiral or helical method. During assembly, an upper part, which is already provided with a roof, is raised, so that further panels or wide strips can be spirally attached thereunder. The upper part has to be rotated about its centre axis during this process. It rests either on driven rollers or on spacers, between which tiltable ram-type presses can engage. Such systems are described, for example, in U.S. Pat. Nos. 2,751,672 and 2,866,261 and 3,182,958.
However, each of these known methods has its disadvantages. According to U.S. Pat. No. 2,751,672 there is required a supporting frame provided with spiral rails on which the plate, which is fastened to small roller bogies is pushed upwards. For this purpose, there are used hydraulic rams, which grip the rolling bogies in a stepwise manner. The entire system is very expensive.
U.S. Pat. No. 2,866,261 makes use of electrically driven, stationary rollers, which are placed on the lowest spiral section of the tank jacket and which serve for rolling the upper tank part upwards, so that further plates can be inserted. In the construction of large and high tanks, which are the only type where this assembly method becomes an economical proposition, the lower edges of the plates of the upper part have to be protected with screwed-on split guide rings. By way of example, the upper part of a tank having a diameter of 29m and a height of 27m weighs approximately 260,000 kg. before the end or last plate is fitted. The transport rollers have to be designed with a correspondingly large diameter of approximately 250 mm. This results in a disadvantageously large distance between the upper part and the lowest jacket section, involving a greater expenditure for the lowering operation. The guide ring parts have to be made separately for each tank diameter; their assembly and disassembly is expensive and also makes the insertion of each additional plate difficult. Furthermore, the running surfaces of the rollers must have sufficient friction; for example, they must be grooved, in order to prevent the load of the heavy upper part from being shifted. However, this also necessitates roller drives of a corresponding strength. If a number of tanks are erected in series for a tank depot of some considerable size, then a further set of electrically driven rollers has to be available for the second tank, so as to make it possible to commence the assembly of the roof on the second tank.
U.S. Pat. No. 3,182,958 describes a further method in which the upper part of the tank is raised during assembly with a fairly large number of lifting devices. These are attached to the lower tank jacket section. The movement is effected via lifting cylinders which are mounted so as to be rotatable in two directions. By this means a foot is lifted and tilted forward until it strikes against a spacing plate. The impacts thus caused can be very violent, particularly when the upper tank part is almost completed. The apparatus is very susceptible to trouble and requires a fair amount of maintenance work. Furthermore, it reacts in a sensitive manner to differences in the level of the bottom plate and the foundation disposed thereunder.